The development of toxicity indexes based on lung lavages requires the exposure of a large number of animals to inhalation of toxic substances. Part of the effort can be reduced by running preliminary tests in vitro using cells derived from the target organ such as alveolar macrophages. Since alveolar macrophages remain differentiated and continue to express cell specific traits during short term culture in vitro, they are particularly suited for the study of the effects of toxic substances on their specific properties. The effects of morpholine were studied on alveolar macrophages in vitro. The effects observed not only agreed with those observed in vivo, but they also gave indications on the mechanisms involved.